The present invention relates to multiplex transmission of voice (talk signals) and data signals, and more particularly to a radio communication apparatus and method suitable for transmission of vital sign signals of a patient such as electrocardiograms, pulse waves, and blood pressures from a mobile station such as an ambulance car to a base station such as an emergency control center and an emergency hospital, while having voice communications therebetween.
Multiplex transmissions by frequency division multiplex and time division multiplex have been used as a technique of multiplex transmission of both data signals such as vital sign signals and voice signals between a mobile station such as an ambulance car and a base station.
As illustratively shown in FIG. 5, with the multiplex transmission by frequency division multiplex, a voice signal is allocated with a band width of 300 Hz to 2 kHz and a data signal such as a vital sign signal is allocated with a band width of 2 to 3 kHz. Under this band width limit, a composite signal of the voice and data signals is transmitted over a carrier wave of a single frequency. However, since the allocated band width is narrow, the voice and data signals cannot be separated completely at a destination station and high frequency components of the voice signal are superposed on the data signal. Therefore, frequency division multiplex is almost impractical for such application.
As multiplex transmissions by time division multiplex, the following techniques, for example, are known.
(1) Japanese Patent Laid-open Publication JP-A-5-37420 entitled "Medical Information Radio Transmission Method"
(2) IEEE TRANSACTIONS ON VEHICLE TECHNOLOGY, VOL. VT-33, NO. 3, pp. 205-213 entitled "C900--An Advanced Mobile Radio Telephone System with Optimum Frequency Utilization"
A multiplexing/demultiplexing method of the medical information radio transmission method described in the document (1) will be explained with reference to FIGS. 6 and 7.
As shown in FIG. 6, vital sign signals and voice signals are multiplexed on a transmission side (mobile station) in the following manner. A plurality of channel signals including a vital sign signal (5) and a voice signal (1) and FM modulated on a subcarrier are divided into time slots. The time slots of the vital sign and voice signals are compressed in time by a half to obtain compressed signals (2) and (4). The compressed signals are combined to form a composite multiplex signal (3). The composite multiplex signal modulates a carrier signal which is transmitted as radio waves. As shown in FIG. 7, the multiplex signal is demultiplexed on a reception side (base station) in the following manner. The received radio wave is demodulated into a composite multiplex signal (3') which is then separated into a modulated voice signal (2') and a modulated vital sign signal (4') of each channel. The modulated voice and vital sign signals are expanded in time by a twofold to obtain the original channel signals (1') and (5'). The subcarrier is demodulated to obtain the original vital sign and voice signals. A voice signal is transmitted from the base station to the mobile station by using a radio frequency different from that used for transmission of the multiplex signal from the mobile station to the base station.
In the case of the radio transmission method described in the document (2), a voice signal is divided into predetermined time slots on the transmission side (mobile station), and each time slot is compressed in time to about 90%. A data signal is directly inserted in vacant time slots formed by the time compression, in units of one block of, for example, four bits of data bursts. A carrier signal is modulated by a composite multiplex signal of the data signal and voice signal, and the modulated signal is transmitted as radio waves. On the reception side (base station), the received radio waves are demodulated to obtain a composite multiplex signal which is then separated into the original data signal and the compressed voice signal. The compressed voice signal is expanded in time to obtain the original voice signal.
The following problems are, however, associated with the conventional multiplex transmission by time division multiplex.
(a) The pass band of a transmission signal over a radio channel is generally in the range of about 300 Hz to 3 kHz. A data signal to be transmitted, such as an electrocardiogram signal, has in some cases very important information outside of the pass band, such as lower than 100 Hz. Therefore, if electrocardiogram data itself is transmitted without any processing, the very important information cannot be transmitted and is unable to be used in practice. No countermeasure for solving this problem is disclosed in the radio transmission method of the document (2).
With the medical information transmission method of the document (1), a subcarrier of about 1.1 kHz is modulated by a vital sign signal through analog FM modulation and then time compression is performed on the mobile station side. On the base station side, the subcarrier is demodulated to obtain the original vital sign signal. Since the subcarrier is modulated through analog FM modulation, the mobile station is required to have an analog FM modulator, a subcarrier signal generator, an analog synthesizer for synthesizing the compressed voice signal and data signal, whereas the base station is required to have an analog FM demodulator and a subcarrier signal demodulating circuit. However, an analog signal processing circuit is generally expensive and difficult to make it compact. Therefore, the transmission/reception apparatus becomes very bulky, heavy in weight, and high in cost. It is also difficult to transport the apparatus and install it at the mobile station, particularly at an ambulance car which has a limited mount space.
(b) In multiplex transmission of a voice signal and a data signal, it is necessary to set a compression factor of the voice signal as high as possible in order to allow the data signal to occupy a large space in the multiplex signal and in order to raise a data transmission rate. On the contrary, in order to have a good sound quality of a reproduced voice signal, it is necessary to set the compression factor as low as possible. An optimum compression factor solving the problem of this trade-off must be set in order to perform good multiplex transmission. However, the above conventional techniques use fixed compression factors of a voice signal and a data signal, irrespective of the characteristics of a transmission signal. Therefore, either the sound quality of a reproduced signal or a data transmission rate is sacrificed or becomes insufficient for practical use at worst.